Individuals with confirmed or clinically suspected SMS, their parents, and/or unaffected siblings will be enrolled in this comprehensive longitudinal natural history study. An NIH interdisciplinary SMS Research Team (SMS-RT) of clinical and basic science researchers, in collaboration with extramural investigators, will conduct comprehensive clinical and molecular analyses to delineate the physiological, developmental (cognitive), behavioral, biochemical, and cellular processes that characterize the syndrome. The protocol aims to: characterize the phenotypic variability, natural history and underlying pathophysiology of SMS; delineate the neurobehavioral phenotype with respect to sleep disturbance, cognition, mood and maladaptive behaviors and sensory processing dysfunction; investigate the physiologic and functional aspects specifically underlying delays in speech/language and motor development; explore genotype/phenotype correlations to identify gene(s) or modifiers that contribute to phenotypic variability; determine potential intervention and therapeutic strategies likely to improve outcome; and evaluate the psychosocial impact of SMS on the family system. Offsite enrollment of two pediatric comparison groups to participate in the Home Assessment of Sleep (HAS) are also enrolled: 1) unaffected siblings/control group; and 2) a DD/ID-comparison group of children with developmental/intellectual disabilities associated with sleep disturbances.